Large low-speed two-stroke self-igniting (Diesel) engines of the crosshead type are typically used in propulsion systems of large ships or as prime mover in power plants. Very often, these engines are operated with heavy fuel oil or with fuel oil.
Recently, there has been a demand for large two-stroke diesel engines to be able to handle alternative types of fuel, such as gas, coal slurry, petroleum coke and the like, in particular gas.
Gaseous fuels, such as natural gas are relatively clean fuels that result in significantly lower levels of sulfurous components, NOx and CO2 in the exhaust gas when used as fuel for a large low-speed uniflow turbocharged two-stroke internal combustion engine when compared with e.g. using heavy fuel oil as fuel.
However, there are problems associated with using a gaseous fuel in a large low-speed uniflow turbocharged two-stroke internal combustion engine. One of those problems is the willingness and predictability of gas to self-ignite upon injection into the combustion chamber and both are essential to have under control in a self-igniting engine. Therefore, existing large low-speed uniflow turbocharged two-stroke internal combustion engines use pilot injection of oil or other ignition liquids simultaneously with the injection of the gaseous fuel to ensure reliable and properly timed ignition of the gaseous fuel.
Large low-speed uniflow turbocharged two-stroke internal combustion engines are typically used for the propulsion of large ocean going cargo ships and reliability is therefore of the utmost importance. Gaseous fuel operation of these engines is still a relatively recent development and reliability of the operation with gas has not yet reached the level of conventional fuel. Therefore, existing large low-speed two-stroke diesel engines are all dual fuel engines with a fuel system for operation on gaseous fuel and a fuel system for operation with fuel oil so that they can be operated at full power running on the fuel oil only.
Due to the large diameter of the combustion chamber of these engines, they are typically provided with three fuel injection valves per cylinder, separated by an angle of approximately 120° around the central exhaust valve. Thus, with a dual fuel system there will be three gaseous fuel valves per cylinder and three fuel oil valves per cylinder with one fuel oil injection valve placed close to a respective gas injection valve so as to ensure reliable ignition of the gaseous fuel and thus, the top cover of the cylinder is a relatively crowded place.
In the existing dual fuel engines the fuel oil valves have been used to provide pilot oil injection during operation with gaseous fuel. These fuel oil valves are dimensioned so as to be able to deliver fuel oil in an amount required for operating the engine at full load on fuel oil only. However, the amount of oil injected in a pilot injection should be as small as possible to obtain the desired reduction in emissions. Dosage of such a small amount with a full size fuel injection system that can also deliver the large amount necessary for operation at full load poses significant technical problems, and is in practice very difficult to achieve and therefore the pilot oil dosage has in existing engines been with a larger quantity per fuel injection event than desirable, especially at medium and low load. The alternative of an additional small injection system that can handle the small pilot amount is a considerable complication and cost up. Further, additional small pilot oil injection valves render the top cover of the cylinder even more crowded.